The present invention is an invention for electromagnetic actuators. A solenoid valve will be described as one example of an electromagnetic actuator. A solenoid valve is equipped with a plunger and a stator core which is disposed to face the plunger, wherein the plunger has a valve element which is proximate to a valve seat, and the stator core has a solenoid coil which is wound therearound. In the solenoid valve constructed in the above manner, an electric current is supplied to the solenoid coil so that the plunger is attracted in a longitudinal direction thereof by magnetic force which is generated between the stator core and the plunger. As a result, switching operation of the solenoid valve is performed. The plunger is required to have high magnetic flux density as a component of the solenoid valve. When a shaft of the solenoid valve reciprocates in an axial direction, the shaft of the solenoid valve slides on a pivot for stabilizing an orbit thereof in the axial direction. When the shaft moves to a side opposite to the stator core of the shaft, the shaft repeatedly collides with another member (for example, a valve seat of a solenoid valve having a plunger and a valve element which are integrally combined with each other). Therefore, the shaft is required to have good wear resistance and good fatigue strength against the repeated impacts. Due to these, in recent years, a plunger having a shaft and an outer member which are separate from each other is produced, wherein the shaft has good mechanical properties and the outer member is composed of a soft magnetic material having strong magnetic properties.
FIGS. 1A and 1B are side views showing a typical structure of a solenoid valve equipped with a plunger having separate members as described above. As shown in the Figures, a solenoid valve is equipped with a plunger 3 and a stator core 4. The plunger 3 has a shaft 1 having a valve element 1a proximate to a valve seat (not shown in the Figures) at one end side of the plunger 3, and has a typically cylindrical outer member 2 at the other end of the plunger 3. The stator core 3 is disposed to face the plunger 3 in a radial direction (FIG. 1A) of the shaft 1 or a longitudinal direction of the shaft 1 (FIG. 1B). A solenoid coil 5 is wound around the stator core 4. In the solenoid valve shown in FIG. 1A, an electric current is supplied to the solenoid coil 5 wound around the stator core 4, so that the plunger 3 is attracted in a longitudinal direction thereof by a magnetic force which is generated between the stator core 4 and the plunger 3. And the direction of the electric current is changed, so that the plunger 3 returns to the initial state by the change of direction of the electric current supplied to the solenoid coil 5 or by a restoring force of a spring (not shown in the Figures). As a result, the plunger 3 moves forward or backward. In the solenoid valve shown in FIG. 1B, an electronic current is supplied to the solenoid coil 5 wound around the stator core 4, so that the plunger 3 is attracted toward the stator core 4 by magnetic force, and the solenoid valve is thereby opened. Supply of the electromagnetic current to the solenoid coil 5 is stopped, so that the plunger 3 returns to the initial state by a restoring force of a spring (not shown in the Figures).
This switching operation of the solenoid valve depends on a magnetic field generated between the plunger 3 and the stator core 4 based on the change of the electric current supplied to the solenoid coil 5. FIGS. 1A and 1B show directions of lines of magnetic force by dotted lines, which are lines of magnetic force generated when an electric current is supplied to the solenoid coil 5. In order to increase a magnetic flux density generated in the above manner and use magnetic fields effectively, in conventional techniques, it was preferable that a nonmagnetic steel be used for the shaft 1 of the plunger 3, so that leakage of magnetic flux be inhibited. For example, a nonmagnetic stainless steel SUS304 of the Japanese Industrial Standards (=JIS) was generally used for the shaft 1. The steel SUS304 corresponds to a steel 304 of the American Iron and Steel Institute(=AISI).
In the above manner, in the case in which a nonmagnetic stainless steel is used for the shaft 1, in the conventional solenoid valve shown in FIG. 1A, the shaft 1 which is nonmagnetic and the outer member 2 are typically composed of a steel, and are bonded integrally with each other by press-fitting and caulking. However, there are various limitations to the plunger 3, for example, limitations to the material of the plunger, shape thereof, and production process therefor. For example, only materials which can be plastically deformed are used for the plunger 3. Extremely precise dimensions are required for the finishing of the inner diameter of the plunger 3, so that production costs are increased. In a case in which a material of the plunger 3 is subjected to plastic working, the material must occupy a space of a predetermined size for plastic working, so that there are limitations in the reduction in size and weight of the plunger 3.
In order to overcome the above limitations, a sintered plunger is proposed for a solenoid valve having a structure shown in FIG. 1A (see Patent Publication 1). In Patent Publication 1, the outer member 2 is composed of a sintered material, and the shaft 1 made of a nonmagnetic steel is fit into an inner hole of a green compact of the outer member 2. Next, the outer member 2 and the shaft 1 are bonded with each other by sintering, wherein sintering of the outer member 2 and diffusion bonding of the outer member 2 and the shaft 1 are performed in one processing. A technique is proposed in which a member has a shaft portion made of a steel, a green compact having a hole portion is made by compacting a powder of an Fe-based alloy or a mixed powder of an Fe-based alloy, the member and the green compact are sintered in the condition in which the shaft portion is fit into the hole portion (see Patent Publication 2).
Patent Publication 1 is Japanese Unexamined Patent Application Publication No. 2000-87117. Patent Publication 2 is Japanese Unexamined Patent Application Publication No. 2000-87114.